PELAGIC SEDIMENTS 347 



The most distinctive chemical and other characters of pelagic 

 deposits thus seem related primarily to their slow accumulation 

 and the resulting approach to chemical equilibrium with the 

 normal open ocean bottom water environment. Even the com- 

 paratively rapid pelagic accumulation of some Globigerind ooze in 

 the equatorial Pacific, at rates of several centimeters per thousand 

 years, results in only the oxidized minerals of iron and tetravalent 

 manganese. The gray diatomaceous sediments of the subarctic 

 Pacific are not highly oxidized. Menard (1955, p. 240) has indi- 

 cated, however, that this is largely glacial-marine and turbidity 

 current deposition. Diatom remains are a distinctly minor element, 

 and as indicated, these deposits also lack the slow accumulation 

 and other features of typical pelagic deposits. 



Many of the more interesting discoveries and advances in 

 knowledge of pelagic sediments are coming from geochemistry, 

 and the stratigraphic evidence of former conditions that is indi- 

 cated in core samples. Much regarding the surface sediments, 

 however, remains little changed from the pioneer work of Murray 

 and Renard (1891). Not only is the Challenger volume on deep-sea 

 deposits a great compilation of data, but much of the interpre- 

 tations seem confirmed. 



The classification and most of the terms of Murray and Renard, 

 though not entirely satisfactory, are sufficiently descriptive and 

 well known not to require time to discuss the original or any 

 modified definitions here. Even their map of distribution covering 

 most of the earth's surface presented much of the major features 

 indicated on the map from Sverdrup et al. (1946, p. 975, Fig. 253). 

 It will be indicated that though the pelagic clay includes great 

 areas with different colors and other characters, the old term "red 

 clay" still seems not entirely inappropriate as a general term, in 

 that it implies the highly oxidized character of all pelagic clays. 



The great volume of calcareous deposits in the oceans presents 

 problems that have been much discussed. The present rate of 

 supply of calcium from the rivers is sufficient to double the large 

 amount in the oceans in about one million years if no calcium 

 were removed, and Revelle and Fairbridge (1957, p. 244) calcu- 

 lated this should result in deposition equivalent to 0.66 g of 

 carbonate per square centimeter per thousand years for an area 



